Solid-state YAG lasers were previously used for a laser welding, cutting, or marking of metals in metal processing. Fiber lasers were recently developed and begun to be used practically in the wavelength region between 1.0 micrometer and 1.1 micrometers. The fiber lasers are paid attention as a replacement of YAG lasers.
Optical reflectivity of metals is very high in these wavelength regions, especially the optical reflectivity of gold, silver, copper, and aluminum are more than 90%. Although iron, nickel, and cobalt metals have relatively low optical reflectivity and suit for laser processing in these wavelength regions, the optical reflectivity of these metals is more than 50%. It must be taken into consideration that a considerable amount of the laser light energy returns to the laser light source, though all of the radiated energy does not return to the laser light source because the surface of the metal workpiece is not always mirror finished surface.
Resonant material of a YAG solid-state laser is crystal and that of a fiber laser is quartz glass which has a lower power damage threshold than crystals. For this reason, a return light to the laser light source and particularly an incident light normal-incidence to the light source have to be avoided. The use of an optical isolator is ultimately effective in cases of fiber lasers wherein the optical polarization plane can always be changed arbitrary. The polarization independent optical isolators are especially useful.
The polarization independent optical isolator is an optical device composed of two optical polarizers made of birefringent crystal and a Faraday rotator. The polarization independent optical isolators are classified into two types with its crystal polarizer type. One optical isolator is using plane plate polarizers, and the other is using wedge polarizers.
In the case of the plane plate type optical isolator, one incident collimated optical beam is separated into two parallel beams after passing through the plane plate polarizer. The two parallel beams propagate through the Faraday rotator whose area of cross section must be at least 2φ, that is twice as large as the diameter φ of the incident beam. On the other hand, in the case of the wedge type optical isolator, the incident beam is separated into two beams, i.e. an ordinary light beam and an extraordinary light beam, at a definite angle θ after passing through the polarizer. The diameter of the Faraday rotator must be φ+d×tan θ, where d denotes the distance between two wedge polarizers. The numerical values of these parameters, namely, beam diameter φ, distance between polarizers d, and the beam separation angle θ determines which type of isolator is more advantageous. It can generally be said that the wedge isolator has an advantage, because the required Faraday rotator cross sections do not increase as a beam diameter increases. In the case of the plane plate type optical isolator, the more power of the fiber laser, the more beam diameter increases practically. In view of this fact, it is expected that there will be an increase in demand for the wedge type optical isolators and that the high durability to the high power fiber laser will be needed for the wedge type optical isolators.